My opponent, Philocat, wanted to debate me on this topic. I definitely look forward to a good one! Good luck to my opponent!

Argument(s)

As long as two love each other (Including two from the same sex), they should be free to marry. Some say this should not be legal coming from religious perspectives. Here is the thing: That is their decision. Maybe in your religion, it is not allowed. But their decision is not affecting you. They might go to hell, but they are willing to live with that.

I have only seen people arguing from a religious perspective, but not everyone. So, I would like to hear Philocat's perspective. I await my opponent's arguments.

Just to note, I will not be arguing against same-sex marriage from a religious perspective as that is not conductive to an interfaith debate.

I have five main arguments against same-sex marriage, the first four explain why it should not be legalised and the fifth explains why it would be an unnecessary legalisation.

The first argument considers those who are already married. Marriage is a contract between a man and a woman overseen and given legal substance by the state. The state has defined it as between a man and a woman for centuries (1). When a two people agree to marry they sign a contract.
These three facts may seem irrelevant, but let me link them together:
When a man and a woman sign the marriage contract, they are legally bonded with each other and the state. However, if the state changes what marriage is, then the contract is changed. What this means is that the husband and wife are now signed up to a different form of partnership than what they agreed to. What right has the state got to force already-married couples to be in a partnership different to what they signed up for?
Legalising same-sex marriage fundamentally changes what marriage is, and as this would change the marriages of millions of people, it violates the contractually autonomous nature of marriage.

My second argument is that same-sex marriage endangers the validity of sacramental marriage. Note that this is not a religious argument as it does not involve accepting the axiom of religion's veracity, only the welfare of those who happen to be religious.
Millions of people have sacramental marriage and coincidentally take the security that is offered by a civil marriage. The reason that people can justifiably be both married sacramentally and civilly is that civil and sacramental marriage have roughly the same meaning.
If same-sex marriage was to be legalised then it would fundamentally differentiate civil marriage from sacramental marriage; this would mean two things:
1. The state is asserting that their definition of marriage is correct, thus implying that sacramental marriages are incorrect. This questions the validity of the marriages of millions of people.
2. If civil marriage was different to sacramental marriage then the state would be less inclined to see sacramental marriage as a valid marriage at all. Again, this could jeopardise the security of sacramental marriages.
The above two consequences of same-sex marriage show that it will negatively affect the millions of people who have sacramental marriage. Therefore it should not be legalised.

My third argument is a variant of the slippery slope argument.
The quintessential maxim of the same-sex marriage lobby is that 'you should be able to marry someone if you love them'. But this would logically entail that one should be able to marry multiple people, or siblings for that matter.
Supporters of same-sex marriage highlight that incest causes defects in offspring, but this would not distinguish between same-sex marriage and incestuous marriage if the latter used contraception. I would also argue that being raised by homosexual parents is not preferable for children, but that's another debate.
To conclude, if we allow same-sex marriage because of love, then this would also allow polygamous and contraception-using incestuous marriages.

Marriage is beneficial to society, children and couples. However, marriage must be respected and valued in order for people to choose to get married, otherwise they will dismiss it as simply a piece of paper or a financial predicament.
Due to this, it is beneficial to society, children and couples to maintain the respect that marriage has.
The issue with same-sex marriage is that legalising it is a direct act of the state changing marriage by bowing under public opinion.
My premise is that a constant institution or concept is more respected, valued and trusted than an institution or concept that is equivocally elastic and malleable under current public attitudes. This comes from our innate psychology; we are less likely to trust in something that is easily changeable because we are not sure what we're putting our trust in. For example, a political party that sticks to its principles despite public opposition can be more easily trusted (and is more respected) than a political party that changes what it stands for in order to please a few people.
It is the same with marriage; if it is changed under pressure from public opinion then it will be ultimately less respected and more suspiciously regarded than if it is not changed. This would mean that less people would get married and society, children and relationships will suffer.
Consequently, legalising same-sex marriage would negatively affect people and society, as doing so involves changing what marriage is.

My final argument explains why same-sex marriage is unnecessary.
The only practical reason why it could be construed to be needed would be to give homosexual couples the financial security, spouse recognition, inheritance and other benefits that heterosexual married couples receive.
However, the ideal solution to this is to allow homosexuals to have civil partnerships that give the exact same legal benefits as a marriage, but does not affect marriage.
I may be missing some other practical benefit that same-sex marriage could give, but at the moment I cannot see anything that civil partnerships wouldn't solve.

These are my five arguments, which together show that legalising same-sex marriage could most likely be harmful to society and those within it; and would bring no tangible benefit.